You may be able to do this by pushing out a file or two into the Firefox folder in c:\Program Files. Haven't tried it myself, but check out this article:

Locking preferences - MozillaZine Knowledge Base

Any luck?

Pushing files around is a bit awkward, I'd rather save that option for a last resort.

Look at cor-el's response in this link (a different pref): https://support.mozilla.com/en-US/questions/808176

and the pref to be set in this Mozilla blog post (last sentence under "Status" section: https://blog.mozilla.com/security/2011/06/16/webgl-graphics-memory-stealing-issue/

Also note that a fix for this is due to be out on 2011-06-21.